The present invention relates to the production of certain volatile elemental metals by means of thermal reduction of their oxides with another metal, where the heat required in the process is obtained from solar radiation.
A variety of processes for producing certain volatile elemental metals by means of thermal reduction of their oxides with another metal have been described in literature, notably in relation to the recovery of elemental zinc from its oxide. In these known processes zinc oxide, iron and carbon are mixed in a suitable reaction vessel, the mixture is heated to an elevated temperature at which the reduction occurs, say about 1500.degree. C., and elemental zinc is recovered in gaseous form from the reaction mixture. Processes of this kind are described, for example, in U.S. Pat. No. 1,925,458, U.S. Pat. No. 1,988,608, FR 583,632, FR 728,928 and DE 452784. In all these known processes there is obtained a gaseous mixture of elemental zinc and carbon oxides with the occurrence of a back reaction in which part of the product zinc and the carbon oxides react with each other with the substantial reformation of the starting zinc oxide ZnO and carbon. Furthermore, the portion of gaseous zinc which does not back-react has to be separated at elevated temperature from the accompanying carbon oxides which further complicate the process.
As was mentioned above, the heat required for the process of the present invention is derived from solar radiation. A major problem associated with this source of energy is its transient nature, being dependent upon both predictable daily and yearly changes, and on unforeseeable factors such as weather conditions (haze, clouds, etc.). A solution to this problem is to be found in using solar energy as process heat and storing the energy for periods of insufficient solar availability. One concept of storing solar energy is based on the utilization of the radiation in solar-driven highly endothermic reactions, and releasing the heat when and where required by the reverse exothermic reaction. In such a case, which will be referred to hereinafter as "a closed loop process", the original products are recovered and are available for recycling. An example of the closed loop concept may be found in the loop of methane reforming, as described by Levitan et al. in Solar Energy Materials, 24, 1-4, pp. 464-477 (1991). Another approach is, to transform solar energy into a manageable, storable energy form, and at the same time conserve natural resources by replacing the process heat conventionally produced from raw materials, by solar energy. The products of such a process are not back-reacted to form the starting materials, but are used as chemical commodities. This type of process will be referred to hereinafter as "an open loop process".